Bearing friction, power loss and starting torque

Bearing friction is not constant and depends on certain tribological phenomena that occur in the lubricant film between the rolling elements, raceways and cages.

Diagram 1 shows how friction changes, as a function of speed, in a bearing with a given lubricant. Four zones are distinguishable:

Zone 1 – Boundary layer lubrication condition, in which only the asperities carry the load, and so friction between the moving surfaces is high.

Zone 2 – Mixed lubrication condition, in which a separating oil film carries part of the load, with fewer asperities in contact, and so friction decreases.

Zone 3 – Full film lubrication condition, in which the lubricant film carries the load, but with increased viscous losses, and so friction increases.

Zone 4 – Full film lubrication with thermal and starvation effects, in which the inlet shear heating and kinematic replenishment reduction factors compensate partially for the viscous losses, and so friction evens off.

SKF model of bearing friction

In the SKF model for calculating bearing friction, the frictional moment, M, is derived from four sources:

M = Mrr + Msl + Mseal + Mdrag

where

Mrr

the rolling frictional moment, and includes effects of lubricant starvation and inlet shear heating [Nmm]

Msl

the sliding frictional moment, and includes the effects of the quality of lubrication conditions [Nmm]

Mseal

the frictional moment from integral seals [Nmm]
Where bearings are fitted with contact seals, the frictional losses from the seals may exceed those generated in the bearing.

When the total frictional moment, M, of the bearing is known, you can calculate the bearing frictional power loss using

Ploss = 1,05 x 10–4 M n

where

Ploss

bearing frictional power loss [W]

M

total frictional moment [Nmm]

n

rotational speed [r/min]

Starting torque

The starting torque of a rolling bearing is defined as the frictional moment that must be overcome by the bearing to start rotating, at an ambient temperature of 20 to 30 °C (70 to 85 °F). Therefore, only the sliding frictional moment and the frictional moment of seals, if applied, are taken into consideration.